An inverter circuit for driving a load such as a motor is a converter between direct current and alternating current. The circuit converts a direct current voltage to an alternating current voltage, and then, energizes the load such as a motor. The inverter circuit for driving an inductive motor includes an IGBT (i.e., insulated gate bipolar transistor) as a switching element and a FWD (i.e., free wheel diode). The IGBT functions as a switching element, and the FWD bypasses the current flowing through the motor when the IGBT turns off so that the current flowing through the motor is not changed by a switching operation of the IGBT. Specifically, a direct current power supply and the motor are connected to each other. When the IGBT for applying the voltage to the motor turns off, the current flowing through the motor flows back via the FWD because of the energy accumulated in an inductance L of the motor. Thus, the motor becomes a state equivalent to a case where the reverse direct current voltage is applied to the motor. Accordingly, since the current of the motor is not shut down rapidly because of the switching operation of the IGBT, the alternating current voltage caused by the switching operation is substantially supplied from the direct current power source. Since the inverter circuit performs the above function, it is necessary to form a diode, which is inversely connected to the IGBT in series. Specifically, it is necessary to form the diode that is inversely connected in parallel to the IGBT, which pairs up with another IGBT.
FIG. 10 is an equivalent circuit diagram of a semiconductor device 90 suitably used for an inverter circuit, which drives a load such as a motor. The device 90 includes an IGBT 90i and a diode 90d, which are inversely connected in parallel to each other.
When the diode 90d in the device 90 is used as a FWD in the inverter circuit, it is important to form a current waveform appropriately in a case where the diode is recovered inversely when the diode switches from an on-state to an off-state.
FIG. 11A shows a circuit for measuring and evaluating the current waveform flowing through the diode 90d. FIG. 11B shows an example of the current waveform.
The circuit for measuring includes two semiconductor devices 90a, 90b, each of which is equivalent to the device 90 in FIG. 10. The IGBT 90ai in the device 90a functions as a switching element. The IGBT 90bi in the device 90b short-circuits so that a waveform of a current Id flowing through the diode 90bd is measured.
As shown in FIG. 11B, when the IGBT 90ai of the device 90a turns off, a circulation current Iif flows through the diode 90bd in the device 90b. When the IGBT 90ai in the device 90a turns on, a current inversely and instantaneously flows through the diode 90bd in the device 90b. A peak of the current inversely flowing is defined as a recovery current Irr. When the circuit is recovered inversely, the power source voltage is applied to the diodes. A product of the voltage and the recovery current Irr is defined as a recovery loss. In general, it is required for a rectifier diode to have a small recovery current Irr and a small recovery loss in case of inverse recovery time so that the current in the diode is recovered gradually in case of inverse recovery time. This recovery of the diode is defined as soft recovery.
In the device 90 in FIG. 10, the IGBT 90i and the diode 90d are formed on different semiconductor substrates or semiconductor chips, respectively. The IGBT 90i and the diode 90d are inversely connected in parallel to each other via an electric wiring. To reduce the dimensions of the device 90, it is preferred that the IGBT 90i and the diode 90d are formed on the same semiconductor substrate.
FIGS. 12A and 12B show the semiconductor device 80 in JP-A-2007-227806. FIG. 13 is a cross sectional view of the device 80.
The device 80 is used for an inverter in a vehicle. The device 80 includes an IGBT cell region and a diode cell region, which are formed on the same semiconductor substrate 1 having a N−conductive type. In the device 80, the IGBT cell region and the diode cell region provide an active device cell area. In the cell area, a first semiconductor region 2 having a P conductive type is formed in a surface portion of the substrate 1 on a principal side. The first semiconductor region 2 provides a channel forming region in the IGBT cell region and an anode region in the diode cell region. An emitter region 3 of the IGBT is formed in the first semiconductor region 2. A gate electrode G in a trench is formed in the IGBT cell region. A structure similar to the gate electrode G is formed in the diode cell region, and is not electrically connected to another part. The structure merely divides the diode cell region into multiple parts. In the IGBT cell region, a second semiconductor region 4 having a P+conductive type is formed in a surface portion of the substrate 1 on a backside. The second semiconductor region 4 provides a collector region. In the diode cell region, a third semiconductor region 5 is formed in a surface portion of the substrate 1 on the backside. The third semiconductor region 5 having a N30 conductive type provides a cathode region. In FIG. 13, a current flowing through the diode cell region is shown as an arrow when the diode functions under a forward voltage. A field stop layer 1a of the IGBT is formed on both of the second and third semiconductor regions 4, 5. The field stop layer 1a has a N conductive type.
In a pad region, a fourth semiconductor region 6 having the P conductive type is formed in a surface portion of the substrate 1 on a principal side. The fourth semiconductor region 6 surrounds the cell area. The first and fourth semiconductor regions 2, 6 are electrically connected to each other together with the emitter region 3 in the IGBT cell region via an emitter electrode wiring E. A pad 8 is formed on the fourth semiconductor region 6 via a LOCOS oxide film 7. The pad 8 provides the pad region for bonding with a wiring. In the periphery area, a fifth semiconductor region 9 having the P conductive type and providing the withstand region is formed in a surface portion of the substrate 1 on the principal side. The fifth semiconductor region 9 surrounds the fourth semiconductor region 6. The pad region and the withstand region provide the periphery area. In the periphery area, the second semiconductor region 4 is formed on the backside of the substrate 1 so that the second semiconductor region 4 extends from the IGBT cell region. The second semiconductor region 4 and the third semiconductor region 5 in the diode cell region are electrically connected to each other via a collector electrode C, which is formed on a whole surface of the backside of the substrate 1.
FIG. 14 shows an example of a voltage-current (i.e., Vf-If) characteristic of a diode, which is used in an inverter circuit in a vehicle when the diode functions along with the forward direction. FIG. 15 shows a relationship between a recovery current Irr and a forward operation voltage Vf when the diode is used for a FWD.
As shown in FIG. 15, the recovery current Irr and the forward operation voltage Vf have a trade-off relationship. Specifically, when the recovery current Irr becomes small, the forward operation voltage Vf increases, as shown in an arrow XVA. On the other hand, when the forward operation voltage Vf becomes small, the recovery current Irr increases, as shown in an arrow XVB.
Thus, it is required for the semiconductor device having the IGBT cell region and the diode cell region that are formed on the same substrate to have a small forward operation voltage Vf and a small recovery current Irr.